Letter of Transmittal:
To:
Dr. Henry, Chemical Engineer PhD, PE
From: Greg Kirton, Chemical Engineering Student - Team Creative
Date: April 21, 2004
This document is an examination of the cost/benefit for the methanol solvent
recovery project in which a waste stream is currently being discarded as hazardous waste.
This document suggests an alternative approach to discarding the waste stream, by
separating the components within the waste stream for savings by using non-hazardous
waste disposal as well as an incentive for 99% pure methanol.
Technical, & Economic Feasibility:
Solvent Recovery Project
Greg Kirton
Team Creative
4/21/04
ENCH 430
Executive Summary:
The purpose of this document is to examine the potential cost benefit from
implementing a distillation column to recover valuable solvent while reducing waste
disposal costs. There is also an incentive to separate the methanol to 99% purity, and
receive a $1 credit per gallon.
The current waste disposal is disposed of as hazardous waste and the cost is
$230,000 per year. Implementing the distillation column to perform the separation, the
hazardous waste cost is eliminated, the operational costs for this design are only $13,000
per year and the waste disposal becomes non-hazardous waste which only costs $35,000
per year. The credit from the 99% pure methanol separation yields $62,000 per year,
which results in a net revenue of $14,000 per year.
The investment for this project is $380,000, and the payback period is 2 years,
using the simple payback method. The total cost savings this design produces is
$245,000 per year. In 5 years, this project will have resulted in a total savings of
$840,000.
Introduction:
A process produces a waste stream of methanol and water that is normally
disposed of as hazardous waste at an expense of $1.15/gallon. The process generates
200,000 gallons per year, resulting in an annual waste disposal cost of $230,000.
However, if the methanol could be separated from the water, the plant could receive a
reduction in waste disposal costs at a rate of $0.26/gallon. The condition for the reduced
costs is the wastewater must contain only 1% methanol. An additional cost benefit is
possible; this benefit is $1 credit per gallon of 99% pure methanol. This document will
examine the technical and economic feasibility of reducing waste costs and recovering
methanol solvent
Design:
The proposed design incorporates a distillation column to perform the separation
as shown in Figure 1.
Methanol
Condenser
Binary Mixture
F= 660 lb/hr
27% MeOH
73% H2O
D = 260 lb/hr
99% MeOH
1% H2O
Distillation
Tower
Water
Reboiler
B= 400 lb/hr
1% MeOH
99% H2O
Figure 1 – Block Flow Diagram of Distillation Process.
The feed entering the distillation column was examined at 27% methanol - 73%
water, temperature is 22oC at a flowrate of 660 lb/hr with a vapor fraction of zero in the
feed. Using an overall balance along with a species balance, the distillate and bottoms
product flowrates can be determined. The equation for the overall balance is as follows:
Overall Balance
where
F – Feed flowrate
B – Bottoms flowrate
D – Distillate flowrate
F=B+D
(1)
Species Balance
XfF=XbB=XdD
(2)
Where
Xf – feed composition
Xb – bottoms composition
Xd – distillate composition
Using equations (1) & (2) the overall balance around the column is complete.
First the reflux ratio must be determined. The reflux ratio was calculated using a VLE
model for the binary system of methanol and water. This thermodynamic data can be
found in Perry’s Handbook for Chemical Engineers. Figure 2 shows the x-y diagram for
methanol-water at 14.7 psi.
Methanol-Water
1.0
0.9
Min Reflux = Xdistillate/Yintercept -1
Xdistillate = 0.99
0.8
Reflux Ratio Used = 1.25*Min
Reflux
0.7
Yintercept = 0.52
0.6
y (mole fraction)
0.5
0.4
0.3
0.2
Program prepared by:
Dr. J. R. Cunningham
Mr. M. B. McGann
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
x (mole fraction)
Figure 2 – VLE Model for Methanol-Water @ 14.7 psi
0.9
1.0
The reflux ratio used for this column was 1.1; this value was based off the
operating reflux ratio given in Perry’s Handbook as typically being 1.1-1.5 times the min
reflux ratio.
In order to determine the column diameter, there are four additional flowrates that
are unknown that can be solved for using equations found in the McCabe-Smith text in
the Distillation chapter. The variables are shown in Figure 3. The equations required to
solve for these variables are as follows:
L = D x Rdmin
V=L+D
L’ = L + (1 – f) x F NOTE: f is the fraction of vapor in the feed
V’ = V – f x F
Condenser
Distillate- 660 lb/hr
MeOH - Water
99% - 01%
V
L
Feed - 660 lb/hr
MeOH - Water
27% - 73%
Reflux
Distillation
Tower
V'
L'
Reboiler
Bottoms- 660 lb/hr
MeOH - Water
01% - 99%
Figure 3 – Block flow diagram of process identifying the L, V, L’, and V’ variables.
Once all of the stream flowrates have been identified, the next step performed was
to select the diameter of the column based on the allowable vapor velocity @ 9” tray
spacing, and the vapor volume flowrate, VVF.
Vapor Volume Flow
where:
R = gas constant
VVF = V’RT/P
(3)
T = temperature
P = pressure
The column diameter was calculated to be 0.8 ft2 and was sized to1 ft2. Using a
VLE program that models the McCabe-Thiele method for stepping off stages, the number
of trays needed to separate the methanol and water at 99% methanol distillate product
purity, and 99% water bottoms product purity was determined to be 20 trays. The
number of trays is based on the composition of the feed, distillate, and bottoms product,
as well as reflux ratio and efficiency.
Next, a more detailed process flow diagram is listed and is accompanied by a
stream summary table which lists the total flowrate for each stream as well as component
flowrate, temperature, and pressure..
Dr Henry comments: this drawing is not clear enough to read easily
Figure 4 shows the process flow diagram. This diagram includes all necessary
components required in the process; each component is given a unique ID number to
identify the unit/component within the report. Table 1 provides technical data to
accompany the process flow diagram. This stream summary table includes stream
information such as temperature, overall flowrate, component flowrate, pressure, and
vapor fraction.
Basis
30 lbmol/hr stream 101
Stream #
101
102
111
Temp (o C)
20
22
64.3
Pressure (psi)
14.7
14.7
14.7
Vapor Fraction
0
0
1
Molar Flow (lbmol/hr)
30
30
17
Component Mole Flow (lbmol/hr)
MeOH
8.10
8.10
16.83
Water
21.90
21.90
0.17
30
30
17
112
60
14.7
0
17
113
60
14.7
0
9
114
60
14.7
0
8
121
100
14.7
1
42
122
95
14.7
20
123
100
14.7
1
20
124
100
14.7
1
20
125
100
14.7
1
20
126
95
14.7
0
22
16.83
0.17
17
8.91
0.09
9
7.92
0.08
8
0.42
41.58
42
0.20
19.80
20
0.20
19.80
20
0.20
19.80
20
0.20
19.80
20
0.22
21.78
22
Table 1 – Stream Summary Table for Distillation Process.
Dr Henry comments: tables are not permitted to go beyond the normal margins
The heat duties for the condenser, reboiler, feed, distillate product, and bottoms
product heat exchanger were calculated using equation (3):
Heat Duty
Q=mCpT
where
Q – heat available
m – mass flowrate
Cp – heat capacity of the fluid
T – Temperature change of the fluid
The heat duties for each heat exchanger are shown in Table 2.
(3)
HX
Condenser
Reboiler
Feed Heater
Distillate Cooler
Bottoms Cooler
Stream #
111
123
102
114
125
Condition
Hot
Hot
Cold
Hot
Hot
o
Flowrate (lb/hr) Flowrate (kg/hr) Cp (kJ/kg C) Tin (oC)
Tout (oC) Qav ailable (kW)
550
247
2.6
65
50
3
760
343
4.18
92
100
-3
660
296
3.96
22
65
-14
260
116
2.6
65
27
3
400
181
4.18
80
27
11
Heat Available =
0
Table 2 – Summary of Heat Duties required for Distillation Process.
Using the Q values found in Table 2 as the heat available, the area of each heat
exchanger was sized using equation (4)
Heat Transfer Area
Q=UAT
(4)
where
Q – heat available
U – overall heat transfer coefficient
– NOTE: Use 500 (BTU/ft2 hr oF) for U when using methanol and water as heat
transfer fluids
Dr Henry comments: give reference
A – heat transfer area
T – temperature change of the exchange fluid
Exchanger #
E-111
E-121
E-112
E-122
E-101
Exchangers
Condenser
Reboiler
Distillate Cooler
Bottoms Cooler
Feed Heater
Type of Exchanger
Spiral Tube
Multiple Pipe
Double Pipe
Double Pipe
Double Pipe
Area
(square feet)
22
29
4
1
10
Table 3 – Heat Transfer Area Necessary to Meet Required Heat Duty
Also included in this design is a storage tank that will hold the methanol that is
separated from the feed to yield a $1 credit per gallon. The volume of this tank is 8000
gallon, which will hold one week’s supply of methanol at the production rate of 260
lb/hr. This tank shall be emptied weekly, but if a problem arises and the tank cannot be
emptied within a week, there is 18% freeboard to allow for greater flexibility in the
fill/empty cycle.
This design required the use of three pumps, a distillate pump, a feed pump, and a
bottoms product pump. The pumps used in this design were sized based on the fluid
density, pressure drop and flowrate demand in which the pump must meet. The pumps
required less than 1kW power, but were each specified at 1kW.
Economic Analysis:
Table 4 lists the individual costs for each heat exchanger needed within the
design. The condenser was the largest heat exchanger cost by far, due to the type of heat
exchanger used. The condenser is a spiral tube heat exchanger, the reboiler is a multipipe heat exchanger, and the feed, distillate, and bottoms product heat exchangers were
all double pipe.
HX
Costs ($1000's)
Feed Heater
$12
Condenser
$60
Reboiler
$21
Distillate Cooler
Bottoms Cooler
$12
$12
Table 4 – Heat Exchanger Costs
Table 5 lists the total capital cost for each component required in this design.
Distillation Tower
Heat Exchangers
Storage Tank
Pumps
$70,000
$120,000
$110,000
$80,000
Total $380,000
Table 5 – Total Capital Investment for Solvent Recovery Project.
Dr Henry comments: this total capital investment doesn’t use the Lang Factor to cover
installation, etc. Don’t forget that.
The operating costs for this design are the utilities: electricity required by the
pumps, the cooling water and steam required for the heat exchangers, and the cost of
disposing non-hazardous waste. The annual non-hazardous waste disposal costs $35,000,
and the annual utilities costs are $13,000.
Dr Henry comments: please itemize the utility costs in a table
The total amount of methanol credits produced per year is $62,000, thus
generating net annual revenue of $14,000. The total annual cost savings of this design is
$245,000, due to the elimination of the hazardous waste disposal cost of $230,000 per
year.
Payback Period
$1,000
$840,000
$800
Revenue ($1000'S)
$600
$400
$200
$0
-$200
-$400
-$600
Year 0
Year 1
Year 2
Year 3
Year 4
Year 5
Figure 5 - Cash flow diagram for process examined for 5 years.
Figure 5 shows the cash flow diagram for this process, with the initial capital
investment shown in Year 0. Based on the total annual savings this design yields, this
project will become profitable in Year 2 using the simple payback method under the
assumption of no interest. The value of this project at the end of Year 5 is shown to yield
$840,000.
Conclusions:
Under the conditions stated in the introduction regarding the $1 credit per gallon
of 99% pure methanol produced, as well as a reduction in waste disposal costs from
$1.15 per gallon, to only $0.26 per gallon, this design can become profitable. Given the
initial investment is only $380,000, the total annual savings of $245,000 which results in
the payback period occurring in Year 2, I conclude this design to be feasible to
implement.